Dense twisted bundle heat exchanger

ABSTRACT

A shell and tube heat exchanger is disclosed that is comprised of a dense hexagonal bundle of tubes with tube spacing maintained by spacers on the individual tubes. Exchanger performance is enhanced by any or all of three types of bends of the tubes: bundle twisting; bundle enlargement at the tubesheets; and shell bending. Referring to FIG.  7,  the spacing enlargement bending is illustrated.

BACKGROUND Of THE INVENTION

One cross-cutting requirement for increased energy efficiency is moreand better heat exchange for energy conversion cycles. This invention isdirected toward higher performing and lower cost heat exchangers.

Shell and tube heat exchangers are generally low in cost, but they havecertain constraints that limit their performance. Good performance isachieved when the thermal boundary layer thickness is very thin on bothsides of the transfer surface. Two conditions necessary for that aregood fluid velocity and small hydraulic diameter. It is also importantthat both of those parameters be reasonably uniform throughout theexchanger. For low viscosity liquids a typical “good” velocity is therange 2 m/s (7 fps) to 3 m/s (10 feet/sec). Smaller diameter tubes canachieve the desired performance on the tube side. However on the shellside there are two serious constraints—the baffles and the tubesheets.Segmental baffles create lower than average velocity zones (low heattransfer) and higher than average velocity zones (high pressure drop).The latter causes the overall average velocity to be decreased to wherepressure drop is acceptable.

Various techniques have been proposed to overcome the disadvantages ofsegmental baffles: rod baffles, twisted tubes, tubes with spacers, etc.Each of those introduces other potential problems.

The tubesheet for each end of the tube bundle has a strength-relatedcode requirement that the minimum tube-to-tube spacing be 1.25 tubediameters. That is equivalent to a minimum gap between tubes of 0.25D(25% of the tube diameter). With straight tubes, that spacing ismaintained throughout the bundle. This constrains the shell-sidehydraulic diameter to be larger than the tube-side hydraulic diameter.Furthermore, the bundle geometry and/or the type of baffle frequentlycauses local regions where there are shell-side gaps larger than thegaps between the tubes. Shell flow will preferentially go to those gaps,where little transfer occurs, and hence reduce velocity in the properlyspaced areas.

SUMMARY OF THE INVENTION

A shell and tube heat exchanger is disclosed that is comprised of ahexagonal bundle of round tubes in triangular pitch configurationinserted in a shell with a tubesheet at each end of the shell, intowhich the individual tubes are inserted. The tubes are spaced from oneanother by a minimum gap of between 5% and 25% of the tube diameter. Amajority of the tubes are bent so as to enhance performance.

The invention contemplates three different types of performanceenhancing bends of the tubes. They can be applied all three together orsingly.

One type of bend is the result of twisting the bundle after it isinserted into the shell. The center tube remains straight, but the othertubes are biased into a helicoidal shape around the center tube, with amodest angle to the center tube axis that increases the further out thetube and the greater the twist. The twisting action causes more tubingto enter the shell, filling it out so as to remove the insertionclearance and the non-circular gaps of the hexagon. Thus the shell-sidehydraulic diameter becomes more uniform without the need for insertionof blocking devices. Also the outer tubes are biased against the shellwall to prevent their vibration.

A second type of bend is tube bundle flaring at the two ends of thebundle. That is required when the tube spacing in the bundle is lessthan 1.25 D, whereas the tubesheet spacing is 1.25 D or greater. Thecenter tube is not required to be bent, but all the other tubes receivean S bend: an outward bend to get them to the required location, then aninward bend to get them back to parallel for tubesheet insertion.

A third type of bend is done to the entire assembly of shell plusbundle. Any desired bundle twisting would first be done. Then the shellplus bundle is bent. This reduces the overall dimension of the finishedheat exchanger and provides additional advantages described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the cross section of a circular shell with aseven-tube (two ring) hexagonal bundle inserted, including threespacers, round tubes, and triangular pitch.

FIG. 2 illustrates the cross-section of a shell with a nineteen-tube(three ring) hexagonal bundle, including seven spacers.

FIG. 3 illustrates the cross-section of a circular shell with athirty-seven tube (four ring) hexagonal bundle plus twelve spacers (noteno spacer on the center tube).

FIG. 4 illustrates a helical wire spacer coiled or wrapped around atube.

FIG. 5 illustrates a helical strip spacer coiled around a tube.

FIG. 6 illustrates the cross section of a seven ring hexagonal bundleinserted into a closely conforming circular shell, where approximatelyevery other tube in the bundle has a spacer, and adjoining spacers arewrapped in opposite directions.

FIG. 7 illustrates a cross section of one end of the bundle plus shell,where the tube spacing is enlarged by tube bending to allow the tubes toenter the tubesheet.

FIG. 8 a illustrates a portion of a seven tube hexagonal bundle withoutinserts that has been twisted.

FIG. 8 b illustrates a portion of a seven tube hexagonal bundle withinserts that has been twisted.

FIG. 9 illustrates a shell and hexagonal tube bundle heat exchangerwherein the tube spacing has been enlarged at each end to fit intotubesheets, and wherein the composite shell plus bundle has been bentinto a coil.

FIG. 10 illustrates a shell and tube heat exchanger wherein thecomposite shell plus bundle has been bent into a serpentine shape by aseries of 180° bends of the shell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a seven-tube (two ring) hexagonal bundle of roundtubes (1-7) is illustrated, inserted into a closely conforming circularshell 8. Three of the tubes (2, 4, 6) in the second ring of the hexagonhave helical spacers 9 attached. The tubes in the bundle are intriangular pitch configuration. The spacers have a thickness of 0.15tube diameter, setting that as the minimum tube-to-tube spacing in thebundle.

Referring to FIG. 2, a three ring hexagonal bundle of tubes isillustrated, where the third ring has three tubes (for example 10, 11,12) on each of the six sides of the hexagon. The 19 tube bundle isinserted into a closely conforming circular shell 13. This bundle hasone spacer 14 on the center tube and six spacers 14 on every other tubein the third ring. The spacers are illustrated as 0.138D thickness, thusmaintaining that tube-to-tube gap throughout the interior of the bundle.

Referring to FIG. 3, a four ring hexagonal bundle of round tubes isillustrated in cross section, where the fourth ring has four tubes (forexample 17, 18, 19, 20) on each of the six sides, there are a total of18 tubes in the fourth ring, and a total of 37 tubes in the entirebundle. This bundle does not have a helical spacer on center tube 22. Ithas three spacers 23 in the second ring, three spacers 24 in the thirdring, and six spacers 25 in the fourth ring, for a total of twelvespacers. “Closely conforming” means there is a slight clearance betweenthe shell ID and the bundle OD, sufficient for the assembled bundle tobe inserted into the shell. That clearance would typically range from 0to 0.6 tube diameters, with the larger clearances in the larger tubecount bundles.

The spacers that maintain the spacing for the tubes in the bundle can beof various configurations. Two preferred configuration are illustratedin FIGS. 4 and 5. In FIG. 4, a wire spacer 26 is disclosed that ishelically wrapped or coiled around tube 27. It is possible to only wrapportions of the tube. However given the amount of performance bending ofthe tubes that is disclosed, and the need to maintain the tube spacingin bent regions, it is preferred that the circular wire be wrappedaround the entire length of the tube except for the two ends where shellfluid enters and exits the bundle. In addition to the winding length,the two other geometrical parameters are the wire diameter and the pitchof the wrap. FIG. 4 illustrates a wire diameter of 0.16 tube diameter,thus fixing the tube-to-tube minimum gap. Also, the pitch is two tubediameters, i.e. each wrap advances two diameters along the tube.

FIG. 5 illustrates a spacer 28 comprised of a strip of rectangularcross-section that is helically wrapped around tube 29. The strip has0.2D thickness and 2.5D pitch as illustrated. This cross sectionprovides more bearing surface against the tube, so as to resist localdeformation. Other cross sections are also possible for the helicallywound spacers, e.g. oval.

With the helical spacers, the pitch should be small enough that thecontact points are frequent enough to maintain tube-to-tube spacing evenwhere the tubes are bent. The pitch should also be large enough that thespacer does not impede good fluid contact with the tube. Helical pitchesfrom 1D to 20D are contemplated, with the range of 2D to 4D generallypreferred.

Other types of tube spacers that are known in the art and that wouldserve in this application include: individual discs or washers spacedalong the tube (e.g. U.S. Pat. Nos. 4,386,456 and 2,774,575); protrudingfins or convolutions that are made as part of the tube, especiallyhelical protrusions (e.g. fluted tubing or grooved tubing) (e.g. U.S.Pat. No. 5,181,560); and highly perforated sheet metal strips betweenthe tubes. For example, every tube in FIGS. 1-3 shown having a spacercould be a fluted tube, with the others being bare smooth tubes.

The general idea of the spacers is that each tube with a spacerphysically touches all of its neighboring tubes (through the spacer).Thus all tubes are fixed in position by physical contact. That is incontrast to tubes inserted through segmental baffles, where there isnormally some clearance between the baffle and the tubes that allowssome vibration. Hence the “spacered” bundle is highly resistant tovibration if the contact is maintained.

FIGS. 1-3 illustrate a “sparse” distribution of spacers, i.e. theminimum density of spacers necessary to ensure every tube is contacted.Every interior tube with a spacer is in contact with its neighboring sixbare tubes. Every bare tube in the interior of the bundle is in contactwith three spacered tubes. No spacered tube contacts another spaceredtube. In this sparse configuration, approximately every third tube has aspacer, i.e. is “spacered”.

Two distinguishable sparse configurations of spacers are possible foreach size hexagon bundle (i.e. number of rings). They differ in whetheror not the center tube (ring 1) has a spacer. In FIG. 1, a two-ringhexagon, the center tube does not have a spacer, and also for the fourring hexagon of FIG. 3. However the three-ring hexagon of FIG. 2 doeshave a spacer on the center tube. It has been discovered to beadvantageous when using sparse spacers to have a spacer on the centertube when the hexagon has an even number of rings, and no spacer on thecenter tube for an odd number of rings. The difference affects theoutermost ring of the hexagon, and when done as disclosed it becomesmarginally more compact and closer to circularity.

It is possible to provide a greater density of spacers in applicationswhere that would be useful. FIG. 6 is one example, a five ring hexagonof tubes where approximately every second tube has a spacer, vs everythird tube with the sparse configuration. Wherever two tubes withspacers are adjoining, the spacers overlap. Therefore if the spacers arehelically wrapped around the tubes, any adjoining spacers must bewrapped in opposite direction. That is illustrated in FIG. 6 by denotingright-hand wrap with R and left-hand wrap with L.

It is also possible to have spacers on every tube. Then it is notpossible to have the spacers nest into one another as shown in FIG. 6,and therefore the spacers must be half the thickness of the desiredminimum tube-to-tube gap.

Enlargement—One key advantage of using the sparse spacers is that theshell-side spacing can easily be reduced below 1.25D, thus achievingdesirable small hydraulic diameter. However the tubes must still beinserted in the tubesheets, which will have spacing of at least 1.25D.That is accommodated as illustrated in FIG. 7. Five tubes are shown incross section where they insert into tubesheet 35. All except the centertube have an S bend, first to enlarge the spacing beyond that of most ofthe bundle, and then to return the tubes to parallel at the new spacing.Two of the tubes have wire wraps 36. The wraps end before theenlargement. Whereas bundle tube densities are contemplated down toabout 1.05 tube diameters of tube-to-tube spacing (5% gaps betweentubes), the generally preferred densities are 1.1 to 1.25 diameters.

Twist—The hexagonal bundle of parallel tubes is assembled with spaceredtubes appropriately distributed among the bare tubes. Then the bundle isinserted into an unbent circular shell. At that point there are twoissues to be addressed—the gaps and the loose outer tubes. To facilitateinsertion, the bundle diameter is slightly less then the shell internaldiameter. Thus there is appreciable clearance between the sides of thehexagon and the shell. These clearances result in shell-side gaps(non-uniform hydraulic diameter). The peripheral tubes along the sidesare only supported from one side, and can move and vibrate.

The prior art discloses measures to deal with these two problems such asinserting blockers in the gaps, tie rods, weld bars, etc. See forexample U.S. Pat. No. 7,117,935.

It has been discovered that for smaller tubes (no more than about ¾ inchdiameter) a simpler technique is possible. The bundle is twisted afterinsertion in the circular shell. Twisting the hexagonal bundle causes itto expand to a more circular shape and better fill the shell. Each tubeexcept the center one is caused to assume a helicoidal position in thebundle, with the outer tubes curving at greater angles than the innertubes. The angle of curvature relative to the bundle axis results in thetube cross section being slightly elliptical in the plane of the bundle,more so at the periphery. The tube gaps are retained (by the spacers).The peripheral tubes are pushed out to contact with the shell wall.Since the tubes are no longer straight, the assembly can much betterwithstand differential thermal expansion and thermal shock. The twistingcan be accomplished by loosely inserting the tubes at each end of thebundle into a tubesheet that has not yet been affixed to the shell, andthen rotating at least one of the tubesheets relative to the other. Thetubesheet(s) are affixed to the shell after rotation, and the tubes arerolled and/or welded into the tubesheets after that (also after shellbending if that is done).

In other words, the bundle of tubes is twisted within the shell by atleast one rotation relative to when the shell axis and all tube axes areparallel, i.e. one tubesheet with tubes is rotated relative to where itwould be in the parallel configuration, and relative to the othertubesheet.

Since bundle twisting causes the tubes to be curved along their entirelength, it is important that the spacers be full length also, except notat the ends of the bundle where the shell side fluid enters and exitsthe bundle.

The degree of twist of the bundle does not need to be very severe. Forexample, a bundle that is one inch in diameter and twenty feet longmight advantageously be twisted about six revolutions. The one-inchbundle would thus undergo one full revolution every 40 inches, for abundle pitch of 40D. A three-inch bundle would only require tworevolutions over twenty feet for the same 40D pitch. The bundle pitchwill generally be in the range of 10D to 200D. FIG. 8 illustrates atwisted seven tube hexagonal bundle, with a bundle pitch ofapproximately 2D (i.e. exaggerated twist for ease of illustration). Notethat the shell-side flow continues to be fully countercurrent to thetubeside flow even with this twisting. That is in contrast toconventional shell and coil designs wherein the shell-side flow flowsacross individual tubes of the coils.

Hexagonal wrap—The disclosed bundle twisting technique is advantageousfor smaller diameter tubes and for smaller hexagon bundles, i.e. no morethan about ten rings in the hexagon. Beyond those constraints anothertechnique is preferred to eliminate the gaps and the loose tubes. Asheet metal wrap is provided around the bundle that is hexagonal inshape, the same size as the bundle. The wrap extends the full length ofthe bundle except the ends where the shell fluid enters and exits. Thepressure is maintained approximately the same on both sides of thehexagonal wrap so it can be constructed from quite thin sheet and stillhas no tendency to balloon out. The bundle is first fitted with thehexagonal wrap, and then that assembly is inserted into the shell. Inorder to prevent flow of shell side fluid outside the wrap, a seal ismade between at least one end of the wrap and the shell wall. For lowershell-side pressure applications, the hexagonal wrap could actually bethe shell, and could also be non-metallic, e.g. plastic.

Shell bending—When the bundle of tubes plus the shell is very long andof small diameter, it is advantageous to bend the shell into shortershapes. Especially preferred is a helical coil shape as illustrated inFIG. 9. When this coiling is done there is a strong tendency for thetube bundle to be biased against the inner shell wall of the bend, thuspossibly leaving large gaps at the outer wall. Thus it is very importantto use a hexagonal bundle and to have properly fitted spacers—eithersparsely distributed, or if two helical spacers are in contact, theymust be oppositely wound. It is also important to twist the bundle, soif there is any oversize gap at the outer wall, the fluid traversing itkeeps contacting different tubes. Prior art examples of helically coiledshells are found in U.S. Pat. No. 4,398,567 and NASA Technical NoteD-5092, May 1969.

The shell bending can also be “interrupted”, i.e. bends interspersedwith straight sections, resulting in serpentine or racetrack shapes—seeFIG. 10. The shell bending provides three important benefits—the benttubes have more support points, and hence are almost impervious tovibration; they will accommodate differential expansion such as due torapid temperature cycling or temperature extremes; and the fluid flow ismore turbulent. The key is to not allow the tube spacing in the bundleto collapse due to the shell bending, and thus lose all the benefit.

One preferred combination is the triple helix heat exchanger, comprisedof helical wrap spacers on some of the tubes in the hexagonal bundlewith sparse spacers; a twisted bundle comprised of helicoidal shapedtubes; and a helically coiled shell. The tube spacers, the bundle twist,and the shell coiling are all helical. This combination can have thesame tube-to-tube spacing in the bundle and the tubesheets, or thebundle spacing can be denser than the tubesheet spacing, thus alsoincluding appropriate enlargement of the spacing by tube bending at eachend of the bundle. Other preferred combinations have only a single bendof the shell (U-shaped shell), or no shell bend at all.

With smaller tube sizes in the range of 3/16 inch to ⅜ inch diameter,and dense spacing less than 1.25D, this geometry can achieve heatexchange surface densities in the range of 50 to 150 ft2/ft3. Thetubesheet spacing will frequently be desired to be larger than 1.25Dwith these small diameter tubes to provide welding room, and that can beaccommodated with the disclosed enlargement technique. The fullycountercurrent flow with no significant interruptions or blockages isresistant to fouling.

The disclosed heat exchange geometry is particularly advantageous inapplications requiring high number of transfer units and similar flowarea for both streams. Examples are feed/effluent exchangers,recuperators, and solution heat exchangers.

1. A heat exchanger comprised of: a. a hexagonal bundle of tubes intriangular pitch configuration; b. helically wound spacers on some ofthe tubes; c. a closely conforming shell containing the bundle; and d. atubesheet at each end of the shell plus bundle; e. wherein at least someof the tubes are bent.
 2. The heat exchanger according to claim 1wherein the spacers maintain tube-to-tube spacing of less than 1.25 tubediameters along most of the bundle, and wherein the tubes are bent ateach end of the bundle to increase the tube-to-tube spacing to thetubesheet spacing which is no less than 1.25 tube diameters.
 3. The heatexchanger according to claim 1 wherein at least some of said tube bendsare due to the bundle being twisted inside said shell, wherebyindividual tubes assume a helicoidal shape.
 4. The heat exchangeraccording to claim 2 wherein at least some of said tube bends are frombending the shell plus bundle.
 5. The heat exchanger according to claim1 wherein the tube bends are due to a combination of bundle twisting andshell bending.
 6. The heat exchanger according to claim 1 wherein saidspacers are sparsely distributed on approximately every third tube, andwherein the hexagonal bundle has an odd number of rings of tubes, andwherein the center tube of the hexagonal bundle has a spacer.
 7. Theheat exchanger according to claim 1 wherein said spacers are sparselydistributed on approximately every third tube, and wherein the hexagonalbundle has an even number of rings of tubes, and wherein the center tubeof the hexagon does not have a spacer.
 8. A heat exchanger comprised ofa circular shell that contains a twisted bundle of heat exchange tubes,plus a tubesheet for each end of the bundle.
 9. The heat exchangeraccording to claim 8 additionally comprised of spacers around some ofthe tubes, and wherein the bundle of tubes is twisted at least onerevolution
 10. The heat exchanger according to claim 9 wherein thespacers are helically wrapped around some of the tubes, wherein for anyadjoining tubes with spacers the spacers are wound in oppositedirection, and wherein the bundle of tubes is hexagonal with triangularpitch.
 11. The heat exchanger according to claim 10 wherein the shellplus bundle is bent into a helical coil.
 12. The heat exchangeraccording to claim 10 wherein the shell plus bundle has at least onebend and at least two straight sections.
 13. The heat exchangeraccording to claim 10 wherein the bundle and the tubesheets have thesame tube-to-tube spacing, and that spacing is at least 1.25 tubediameters.
 14. The heat exchanger according to claim 10 wherein thetube-to-tube spacing of the bundle is less than the tube-to-tube spacingof the tubesheets, and additionally comprised of tube bends at each endof the bundle to transition from the one spacing to the other.
 15. Aprocess for manufacturing a heat exchanger comprising: a. assembling ahexagonal bundle of parallel tubes; b. inserting said bundle into acircular shell; c. loosely inserting the tubes into tubesheets at eachend of the bundle; d. twisting one of the tubesheets at least onerevolution relative to the other tubesheet; e. attaching the tubesheetsto the shell; and f. attaching the tubes to the tubesheets.
 16. Theprocess according to claim 15 additionally comprising placing helicallywrapped spacers on some of the tubes before step a.
 17. The processaccording to claim 15 additionally comprising bending the shell betweensteps e. and f.
 18. The process according to claim 15 additionallycomprising bending the tubes at each end of the bundle to matchtubesheet spacing, between steps b. and c.
 19. The heat exchangeraccording to claim 2 wherein at least some of said tube bends are due tothe bundle being twisted inside said shell, whereby individual tubesassume a helicoidal shape, and wherein said heat exchanger has nobaffles for the tubes.
 20. The heat exchanger according to claim 1wherein at least some of said tube bends are from bending the shell plusbundle, and wherein said heat exchanger has no baffles for the tubes.